Masonry

Masonry refers to construction by stone or brick “masonry units” with mortar, and is divided into two types:

Exterior and Interior

Walls come in various configurations, and may be load bearing or non-load bearing

Hollow, Solid and Framed walls

Single and Double-wythe walls

Brick Veneer wall

Cavity walls and masonry-bonded hollow walls

A hollow masonry unit is defined as having a net cross-sectional area in every plane, parallel to the bearing surface, less than 75 percent of the gross cross-sectional area in the same plane

Bricks may be made of clay or cement. Clay bricks may be hollow or solid, or act as tiles in walls or panels

Manufacture of Bricks

Three types of clays are use in brick making:

•Surface clays

•Shales

•Fire clays

Surface Clays are found near the surface in unconsolidated, unstratified material and have high oxide content (10-25%)

Shales are hard material, insoluble in water, and must be ground with added water to become plastic

Fire Clay has uniform physical characteristics and chemical composition, can withstand high temperatures, and has low (2-10%) oxide.

Soft clay is squeezed through dies of the desired shape, then cut to length and fired at high (900-1000 C) temperatures, giving them their characteristic red color

Bricks are formed in three different ways:

•Stiff-mud process

•Soft-mud process

•Dry press process

In the stiff-mud process, a continuous band of clay is forced through a die and cut into bricks

The soft-mud process is best for clays containing too much water; ground clay is hydraulically pressed into a mold

The dry press process is best for clays having little plasticity

The moist bricks are dried at temperatures of 110-300 F for 24-48 hrs.

After drying, they are baked in kilns at temperatures up to 2400 F for 40-150 hours.

After firing, bricks go through a cooling cycle lasting up to 72 hours

Bricks are classified according to their use, such as

•Building (common) brick

•Facing brick

•Floor brick

•Paving brick

Bricks are graded according to their durability:

Grade SW – strongest and lowest maximum water

Grade MW – moderate weathering

Grade NW – Negligible weathering

Grade SW is used where high frost resistance is desired, e.g. below ground level

Grade MW is best or dry climates, e.g., Southern California

Facing Brick is manufactured in three types

FBS – face brick standard, for general use

FBX – face brick extra for high precision is required

FBA – face brick architecture, highest grade; tight tolerances, uniform color

Bricks and tiles are designated by their nominal dimensions, which includes the distance to the center of the mortar joint.

Mechanical testing of Masonry Units

Important Physical Properties of Brick:

•Color

•Texture

•Size

•Density

Important Mechanical Properties of Brick:

•Compressive Strength

•Modulus of Rupture

•Modulus of Elasticity

•Absorption

•Thermal Conductivity

•Fire Resistance

A good brick should absorb less than 20% of water by weight

Water absorption affects the durability of brick. Hard bricks absorb less than 10% water

Absorption is defined as the weight of water absorbed after 24h in cold water divided by the dry unit weight

The saturation coefficient or C/B ratio, is (absorption after 24h in cold water)/(total absorption after boiling for 5h) X 100, or

10013

12 WW

WW

The initial rate of absorption is defined as:

(Dry weight of brick after 1-min. in 1/8” water minus the dry unit weight)/(length of the unit x width of the unit) times 30

BL

WW

30)( 12 Absorption is measured in g/min

per 30 in2

Very dry bricks will have high suction values, and should be thoroughly wetted 3-24 hours prior to use to prevent excessive drying of the mortar

Concrete Masonry Units

Aka: cinder blocks, hollow blocks, concrete blocks

Concrete blocks are made from very dry mixtures of:

•Portland Cement (Type I)

•Aggregate

•Water

Concrete blocks come in three weight classifications:

•Normal weight: 125 pcf when dry

•Medium weight: 105-125 pcf

•Lightweight: 85-105 pcf

Load-bearing masonry units come in two types:

Type I: moisture-controlled

Type II: non-moisture controlled

The primary goal of moisture-content grading is to limit the shrinkage of the units from moisture

loss

•Water absorption of a unit is frequently taken as a measure of its durability.

•Total absorption is measured following a 24 hour period of immersion.

•Linear shrinkage is the change in length of a unit from a wet to a dry condition, and is limited to 0.065%.

•The amount of shrinkage depends on the moisture content in the units and the humidity. The greater the moisture content, the higher the shrinkage when drying. Since shrinkage increases with moisture content, blocks should not be wet prior to placement.

Water Absorption and Shrinkage

Gross area: actual width X actual length

Net area: gross area X percentage of solid

Percent solid: (net volume/gross volume) X 100

Concrete building bricks are manufactured in two grades and two types, based on strength and absorption requirements:

•Grade N (types I and II)

•Grade S (types I and II)

Mortar, Grout and Plaster

Mortar binds blocks in place. Made of:

•Cement

•Lime

•Sand

•Water

Grout is a high-slump concrete that fills cores or voids giving strength

Four grades of mortar: M, S, N and O

M: highest cement content (strongest)

S: highest hydrated lime content (weakest)

Lime can be hydraulic or non-hydraulic, depending on its composition. Adding silica improves its hydraulic properties

Compressive strength of lime mortar is generally in the range of 100-400 psi at one year. The tensile strength varies from 40-150 psi.

Mortars made with finer sand have higher strength

The addition of lime makes the mortar “fat” or “rich”, more plastic and easier to work with

Mortar is described by its properties (above left) or proportions (below)

Grout and its Uses

The purposes of grout are:

•To bond individual units

•To bind reinforcement to the masonry

•To increase the volume, and density, improving load bearing and fire resistance

Grout needs to be thick enough to have high strength, but fluid enough to fill all the voids in the wall

Plaster is a mixture of cement, lime and sand for finishing exterior or interior walls. Stucco is plaster used on exterior walls

Framed walls are covered with heavy felt and wire, then three layers of stucco: the scratch coat, the brown coat and the finish coat

Properties of Masonry

Important properties of masonry are:

•Density

•Compressive strength

•Flexural strength

•Modulus of elasticity

•Durability

•Thermal conductivity

Compressive strength depends on:

•The strength of the units

•The mortar mix proportions

•Grouting

•Workmanship

•Type of bond

Compressive strength of walls can be predicted from the strengths of the masonry prisms (right), two or more units bonded with mortar with or without grout. Strength decreases with increasing height.

Compressive strength of masonry walls based on unit strength and type of mortar

Modulus of elasticity may be estimated from the compressive strength of the wall

Above: clay brick walls, cement-lime mortar

Thermal Conductivity: the time rate of heat flow through a unit area of material, from one surface to the other, per unit temperature difference between the two surfaces

x

TDJ

X

T1 T2Direction of heat flow

Heat transfer depends on the difference in temperature between the two sides, and the ability of the material to conduct heat. Heavy masonry walls discourage heat flow, while cinderblocks do not.

Reinforced Masonry

Efflorescence is the migration (leaching) of white crystals (salts) to the surface of the brick work. This occurs in wet-dry cycles.

To prevent efflorescence, walls should be kept as dry as possible, and good drainage should be provided

Methods of building walls

Methods of bonding walls

Types of Joints